Fastify High-Performance API Patterns

Description

Build and review Node.js APIs with Fastify - app setup with Pino logging and graceful shutdown, route schemas for body, params, query and response with TypeBox and the type provider (validation via ajv, fast serialization, typed requests), the plugin system and encapsulation (fastify-plugin vs scoped plugins, decorators and their TypeScript declarations), lifecycle hooks (onRequest, preValidation, preHandler, onSend, onClose) and returning replies from async hooks, setErrorHandler for validation and application errors, per-request logging, and testing with app.inject(). Use whenever the project depends on fastify, the user builds or migrates an API to Fastify, adds plugins, hooks, schemas or auth, sees "Reply was already sent", decorators missing in routes, fields disappearing from responses, or asks Fastify vs Express.